1,721,083 research outputs found
Surface layer characteristics and SVAT modelling of a fetch-limited forest
Full text linkThe work in this thesis concerns data evaluation and soil-vegetation-atmospheretransfer (SVAT) modelling of measurements taken in and above a temperate beech forest canopy. The SVAT modelling framework used here has been a mixture of an electrical network analogy (single-layer model) and an analytical type of scheme where vertical gradients within the canopy are captured with continuous functions, which can be integrated to the ecosystem level. The beech forest site is fetch limited and influence on forest mast measurements from upwind farmland was investigated. The influence was studied (1) by analyzing ratios of friction velocity taken at different heights in the mast as a function of fetch length and (2) by studying the flux-profile relationship of wind speed and momentum flux, and temperature and sensible heat flux, respectively. The flux-profile relationships also yielded information on the nature of the roughness sublayer which is the lowermost part of the surface layer. The results indicated that the forest site is influenced by the upwind conditions via internal boundary layers. The forest flux-profile relationships were interpreted in terms of aerodynamic resistances. Additional measurements yielded information on the viscous sublayer resistance for heat. Total atmospheric resistances for sensible heat and momentum were approximately equal. Regarding the modelling of soil-vegetation-atmosphere interaction, the focus was on carbon dioxide exchange. A simple model was developed where leaf measurements taken at ambient conditions were used to construct a mean canopy light response curve, which was integrated to yield the carbon dioxide uptake into the tree crowns. In order to compare with the eddy-correlation measurements of net ecosystem exchange, models and measurements of the ecosystem respiration were included. Agreement between the eddy-correlation based estimate and the leaf-measurement based scheme was generally good. The radiation scheme for the integration (upscaling) does not include a distinction between direct and diffuse light. Together with a simple model for the atmospheric resistances, temperature measurements and water vapor fluxes were analyzed to give estimates of mean canopy resistance. The derived estimates were compared to a canopy scale version of the optimal stomatal conductance hypothesis, which regards water lost via transpiration as a cost when assimilating carbon. An approach which calculates canopy photosynthesis analytically is also presented. The work in this thesis represents an attempt to derive simple but accurate formulations for single-layer models of trace gas and heat exchange in forested areas
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Sudden distortion of turbulence at a forest edge
Full text linkDellwik et al. (2013) presented data from a forest edge experiment based on two meteorological towers instrumented with sonic anemometers. The experiment was performed at a dense edge of the Tromnæs Forest, which is a 24 m tall mature beech stand on the island Falster, Denmark. The topography at the site is flat. The towers were placed approximately 1.5 canopy heights upwind and downwind of the edge, respectively, and were two canopy heights tall. For near-neutral, near-perpendicular flow towards the edge, one finding concerned that although the wind speed gradients were similar before and after the edge, the momentum flux was strongly reduced above the canopy. This is contrary to the results by standard Reynolds' averaged Navier-Stokes models that predict an overshoot of the momentum flux. Further, a reduction of the vertical variance of the flow was largely compensated by an increase in the lateral variance, whereas the streamwise variance remained approximately constant. This result is in contrast to the predictions by homogeneous rapid distortion theory. We apply and develop an alternative framework based on inhomogeneous rapid distortion theory, also called blocking, in combination with the turbulence model by Mann (1994), and investigate whether this model can predict the observed changes of the flow. The presented results are relevant for understanding the rapid changes of turbulence in the heterogeneous landscape
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